If it helps, there is quite a bit of information about weapons used during the Vietnam War, provided by the late Lee Russell, a technical advisor during Season 1 of Tour of Duty, on the TourofDutyInfo webpage:

--Previous Message-- : Ar-15s were made by Colt Industries in : Hartford Connecticut. During the early days : of Vietnam War, the SP1 model was designed : and put into production without proper : testing in the Vietnamese climate. It was a : failure as it often jammed during fire : fights. Later on, the m16a1 was designed. : This consisted of a few upgrades including a : Forward Fire Assist on the upper receiver. : It acted as "button" that would : help with jamming. : : Later on, the smaller Xm177e1s were : designed and given to Commando and tank : units. : : --Previous Message-- : Lately I have been interested in the : firearms : shown in Tour Of Duty episodes and always : thought that all M-16 rifles were made by : Colt who bought the rights and ownership : from Armalite and Eugene Stoner (who : designed it). Apparently while Googling to : learn more about the M-16 rifle it seems : that Bushmaster also made them during the : Vietnam war. : : I would like to know the facts from members : on here about what is correct. By the way : what is the best M-16 rifle (or AR-15 rifle) : to own today (as everyone makes it now)? as : I am looking to purchase an AR-15. Thank you : and TOD is one of my favorite shows of all : times. : : Sidney : Dartmouth, Nova Scotia : Canada : : :